Optical disk data storage systems provide the capability to store large quantities of data on a disk. The data is accessed by focusing a laser beam onto the data layer of the disk and then detecting the reflected light beam. Various kinds of systems are known. In a ROM (Read Only Memory) system, such as a compact disk system (CD-ROM), data is permanently embedded as marks in the disk at the time of manufacture of the disk. The data is detected as a change in reflectivity as the laser beam passes over the data marks. A WORM (Write-Once Read-Many) system allows the user to write data by making marks, such as pits, on a blank optical disk surface. Once the data is recorded onto the disk, it cannot be erased. The data in a WORM system is also detected as a change in reflectivity. There are also erasable optical data storage systems, such as phase-change and magneto-optic (M-O) systems. While phase-change systems also read data by sensing a change in reflectivity, M-O systems read data by measuring the rotation of the incident polarization caused by the M-O media. In all of these systems, the focusing lens for the optical beam is located away from the disk.
The data density in optical disk drives is determined by the size of the recorded marks or pits on the disk, which is limited by the diameter of the focused laser spot on the disk. This spot size is the same as the diameter of the focused optical beam, also called the beam waist size. The waist size of a focused light beam is given approximately by .lambda./2NA, where .lambda. is the wavelength and NA is the numerical aperture of the lens. The waist size can be reduced by either using shorter wavelength lasers, such as blue lasers, or by employing higher NA lenses.
Another way to reduce the spot size is through near-field optics, in which a physical aperture is formed which allows light to be transmitted only through the aperture. The optical spot size is determined by the physical dimensions of the aperture, provided operation is in the near-field regime, i.e., closer to the aperture than a fraction of a wavelength. Typically, near-field optics is done by using a tapered optical fiber with metallization on the sides. Since the aperture dimensions can be much less than a wavelength, much smaller spots and therefore higher areal densities can be achieved. A near-field scanning optical microscope (NSOM) has recorded in M-O films at densities of 45 GB/in.sup.2, corresponding to a bit size of about 0.12 .mu.m, as described by Betzig et al., "Near-field magneto-optics and high density data storage", Appl. Phys. Lett 61(2), Jul. 13, 1992, pp.142-144. In imaging modes, resolution down to 200 .ANG. has been obtained. One problem with NSOM is its poor optical efficiency. Only 10.sup.-6 -10.sup.-3 of the light coupled into the fiber makes it out the end. In addition, the collection efficiency in reflection is so poor that the NSOM approach is used only in transmission.
The solid immersion lens (SIL) reduces the spot size by using a high NA lens made of high index of refraction (n) material. A SIL, which is made in the shape of a hemisphere, is described by S.M. Mansfield et al., Optics Letters, Vol. 18, 1993, page 305. The use of a hemispherical SIL for an optical microscope has been described in U.S. Pat. No. 5,004,307, where the spherical surface of the SIL faces an objective lens for receipt of the laser light and the planar surface of the SIL passes through the geometrical center of the spherical surface. As shown in FIG. 1A, a hemispherical SIL has a thickness r, where r is the radius of the sphere. The NA is increased by a factor of n due to the wavelength reduction in the lens. U.S. Pat. No. 5,125,750 describes an optical disk drive with a conical section of a hemispherical SIL supported on a conventional air-bearing slider of the type used in magnetic recording disk drives. The conical section of the hemispherical SIL is held within the slider by springs so that the flat surface of the SIL faces the disk surface.
Another type of SIL, referred to as a "superhemispherical" SIL, is shown in FIG. 1B. This type of SIL includes a partial spherical section and has an overall lens thickness greater than the r thickness of a hemispherical SIL. The superhemispherical SIL has a thickness r(1+1/n), where r is the radius of the partial spherical section. A focused spot will be obtained at the planar surface or base of the superhemispherical SIL when the incident rays are converging toward a point located a distance nr below the center of the sphere. The incoming converging rays are refracted at the surface of the partial spherical lens section, resulting in an increased effective incident angle .theta.. In the superhemispherical SIL, the NA is increased by a factor of n.sup.2, as compared with an increase by a factor of only n in the hemispherical SIL. IBM's U.S. Pat. No. 5,497,359 describes an optical disk data storage system wherein the lens section and body of the air-bearing slider are formed of the same material to thus have the same index of refraction and together form a superhemispherical SIL.
In both the hemispherical and superhemispherical SILs, the small spot exists only within the high index of refraction material because the high angle rays will be internally reflected at the base of the SIL. However, these rays can be coupled via their evanescent fields to the optical disk if the disk is placed less than a wavelength distance from the planar surface of the SIL. Thus, the SIL can be used to increase the storage capacity of an optical disk by the square of the spot size reduction, but only if the SIL can be kept much closer than one wavelength from the disk. The small spot can then be transmitted across a small air gap to the optical disk via evanescent coupling, within the near-field regime. With a red laser (.lambda.=670 nm) and an index of n=2 for the SIL, a spot size of about 0.16-0.2 .mu.m can be obtained. The SIL has excellent efficiency, of the order of 1, and can be operated in reflection, but cannot go to as small a spot size as can the NSOM.
Another approach to near field optical data storage is described in IBM's U.S. Pat. No. 5,602,820. In this approach a sharp tip is used as a localized "antenna" or scatterer of incident light such that the dipole-dipole coupling between the tip and a bit on the storage medium interact to change the amplitude and/or phase of the reflected light. Interferometry techniques are then used to sensitively detect this change of amplitude and/or phase. The optical efficiency of this technique depends strongly on the intensity of the optical beam impinging on the scatterer, leading to an efficiency proportional to NA.sup.2, where NA is the numerical aperture of the lens that focuses incident light onto the sharp tip.
What is needed is an optical disk drive that takes full advantage of near-field optics and the benefits of high NA SILs to achieve a small spot size and thus an increased bit density.